[meteorite-list] Mars Express Discovers Water Supersaturation in the Martian Atmosphere

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Sun, 2 Oct 2011 20:53:13 -0700 (PDT)
Message-ID: <201110030353.p933rDi7008197_at_zagami.jpl.nasa.gov>

http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=49342

ESA orbiter discovers water supersaturation in the Martian atmosphere
European Space Agency
29 Sep 2011

New analysis of data sent back by the SPICAM spectrometer on board
ESA's Mars Express spacecraft has revealed for the first time that the
planet's atmosphere is supersaturated with water vapour. This surprising
discovery has major implications for understanding the Martian water
cycle and the historical evolution of the atmosphere.

Although numerous spacecraft have visited Mars over the past half a
century, very few direct measurements of the vertical structure of the
planet's atmosphere have been made. Since most of the spacecraft
instruments have looked down at the surface, it has only been possible
to infer the horizontal distribution of gases in the atmosphere, leaving
the question of how water vapour is being mixed into the atmosphere
almost unexplored.

This lack of direct measurements has meant that descriptions of the
vertical distribution of water vapour ??? a key factor in the study of
Mars' hydrological cycle ??? has generally been based upon global climate
models.

This gap in the data has now been addressed by the SPICAM (Spectroscopy
for Investigation of Characteristics of the Atmosphere of Mars) imaging
spectrometer on Mars Express.

The instrument can be used in occultation mode, when it studies light
from the Sun that has passed through the planet's atmosphere just after
sunrise or before sunset. The measurements can then be analysed to
generate vertical concentration profiles for several atmospheric
constituents, including water vapour.

Surprising new results, based on SPICAM data obtained during the
northern spring and summer, indicate that the vertical distribution of
water vapour in the Martian atmosphere is very different from previous
assumptions.

Writing in this week's issue of the journal Science, an international
team led by Luca Maltagliati of the Laboratoire Atmosph??res, Milieux,
Observations Spatiales (LATMOS) in Guyancourt, France, describe SPICAM
observations at infrared wavelengths that for the first time provide
evidence for the existence of supersaturated water vapour on Mars.

Supersaturation

The atmosphere of Mars holds 10 000 times less water vapour than that of
Earth. However, water vapour is a very dynamic trace gas, and one of the
most seasonally variable atmospheric constituents on Mars.

Under normal conditions on Earth, water vapour condenses around tiny
dust or aerosol particles or salts when the atmospheric temperature
drops below a certain "dew point". The atmosphere is then said to be
"saturated", since it cannot hold any more moisture at that temperature
and pressure. Any water vapour in excess of the "dew point" will
normally condense to form droplets or icy crystals.

However, supersaturation may occur when some of the water vapour remains
in the atmosphere, instead of condensing or freezing. When condensation
nuclei (assumed to be dust aerosols on Mars) are too rare, condensation
is impeded, leaving substantial amounts of excess vapour.

Until now, it was generally assumed that such supersaturation cannot
exist in the cold Martian atmosphere: any water vapour in excess of
saturation was expected to be converted immediately into ice. However,
the SPICAM data have revealed that supersaturation occurs frequently in
the middle atmosphere ??? at altitudes of up to 50 km above the surface ???
during the aphelion season, the period when Mars is near its farthest
point from the Sun.

Extremely high levels of supersaturation were found on Mars, up to 10
times greater than those found on Earth. Clearly, there is much more
water vapour in the upper Martian atmosphere than anyone ever imagined.
It seems that previous models have greatly underestimated the quantities
of water vapour at heights of 20???50 km, with as much as 10 to 100 times
more water than expected at this altitude.

"The vertical distribution of water vapour is a key factor in the study
of Mars' hydrological cycle, and the old paradigm that it is mainly
controlled by saturation physics now needs to be revised,"/ said Luca
Maltagliati. /"Our finding has major implications for understanding the
planet's global climate and the transport of water from one hemisphere
to the other."

"The data suggest that much more water vapour is being carried high
enough in the atmosphere to be affected by photodissociation,"/ added
Franck Montmessin, also from LATMOS, who is the Principal Investigator
for SPICAM and a co-author of the paper.

"Solar radiation can split the water molecules into oxygen and hydrogen
atoms, which can then escape into space. This has implications for the
rate at which water has been lost from the planet and for the long-term
evolution of the Martian surface and atmosphere."

The new paper analyses SPICAM data obtained when the Martian atmosphere
is relatively dust-free. The absence of dust enables the instrument to
measure the vertical profile to within 10 km of the planet's surface.
The supersaturation levels are likely to plummet in the southern summer,
when dust storms inject large amounts of aerosols into the atmosphere,
increasing the supply of condensation nuclei.

Reference publication

Evidence of Water Vapor in Excess of Saturation in the Atmosphere of
Mars/", by L. Maltagliati, F. Montmessin, A. Fedorova, O. Korablev, F.
Forget, and J.-L. Bertaux, published in the 30 September 2011 issue of
Science.


Contacts

Luca Maltagliati
Laboratoire Atmosph??res, Milieux, Observations Spatiales (LATMOS)
78280 Guyancourt
France
Email: luca.maltagliatilatmos.ipsl.fr
Phone: +33 1 80 28 52 82

Franck Montmessin
Laboratoire Atmosph??res, Milieux, Observations Spatiales (LATMOS)
78280 Guyancourt
France
Email: franck.montmessinlatmos.ipsl.fr
Phone: +33 1 80 28 52 85

Olivier Witasse
Mars Express Project Scientist
Research and Scientific Support Department
Directorate of Science & Robotic Exploration
ESA, The Netherlands
Email: owitasserssd.esa.it
Phone: +31 71 5658015
Received on Sun 02 Oct 2011 11:53:13 PM PDT